Glioblastoma (GBM) is the most common and malignant brain tumor in adults and also accounts for approximately 10% of pediatric CNS tumors. Despite very aggressive disease management, that usually includes surgery, chemotherapy and radiotherapy, GBM is a fatal disease with median survival time of only 12-15 months. Novel approaches and molecular targets for GBM are, therefore, urgently needed. The discovery of microRNAs (miRNAs), small regulatory RNA molecules that cause post-transcriptional down-regulation of gene expression, truly revolutionized the field of cancer biology. It suggested an entirely new layer of gene regulation that might be involved in progression and maintenance of human neoplasia. Our work over the past 8 years focused on miRNAs that contribute to gliomagenesis, and today we have mounting evidence indicating that GBM growth and invasiveness are closely regulated by miRNAs. Importantly, microvesicles released by GBM (which may represent a means of communication between the tumor and its intracranial environment), contain large amounts of miRNA regulators, including the key oncogenic miRNAs. We hypothesize that these molecules are taken-up by normal cells surrounding the tumor, and have significant effects on the physiology of these cells. MicroRNA-mediated regulation of gene expression in the recipient cells may lead to transformative events in the brain cells, serving either protective or, ultimately, tumor growth-supportive function. To validate this hypothesis, we will: 1) characterize the repertoire of intracellular versus extracellular/released RNA in human and mouse GBM cells, and intracellular RNA in normal brain cells that constitute GBM microenvironment; 2) investigate miRNA transfer between GBM and normal cells in co-cultures in vitro, and its functional effects on the phenotypes ofthe recipient cells, and 3) investigate whether miRNA transfer exists between the xenograft GBM models and normal brain cells in animals in vivo. The proposed work promises to yield significant new insights into the biology of glioma and more generally- miRNA-mediated crosstalk between different cell populations in the brain.